The assembly of two electronic components by the so-called “flip-chip” thermocompression technique usually comprises forming electrically-conductive solder balls on a surface of a first electronic component and on a surface of a second component according to a predetermined connection pattern. The first component is then placed on the second component to place their respective solder balls in front of one another, after which the assembly is pressed and heated. The balls placed into contact then deform and melt to form electric connections perpendicular to the main plane of the electronic components, generally in the form of a wafer.
Although “flip-chip” hybridization has many advantages, two problems are however posed for this type of hybridization.
First, a recurrent problem in this type of hybridization lies in the fact that, when no specific measure is taken, the ball surface oxidizes, which creates electric connections of poor quality between the hybridized components. Indeed, not only are metal oxides very poor electric power conductors, but also does the oxide present at the surface of the balls oppose the blending of the balls during the hybridization.
To avoid for balls to be thermo-compressed while they have at their surface a native oxide layer, that is, an oxide layer obtained by the natural oxidation of the metal when it is contact with oxygen, the balls are usually submitted, prior to the hybridization, or during the hybridization, to deoxidizing agents, commonly called “deoxidizing flux” which dissolve the oxide layer. The deoxidizing flux usually is an acid, such as for example benzoic or carboxylic acid. Reference may for example be made to documents JP 2012077214, EP 1 621 566, or U.S. Pat. No. 6,197,560 for examples of deoxidizing fluxes.
Then, the vertical interconnects obtained by the hybridization are sensitive to thermal stress, and this, all the more as the first and second components are made of different materials. Indeed, the components most often have different thermal expansion coefficients, so that under the effect of a temperature variations, the interconnects are submitted to a shearing which embrittles them and breaks them.
To increase the thermo-mechanical reliability of a hybridized assembly and provide a protection of interconnects against the environment, it is generally provided to fill the space separating the two components with a resin layer known as “underfill”, the action of filling this space being known as “underfilling”. The shearing forces are thus distributed all over the layer separating the two hybridized components, and no longer on the interconnects only, the latter being thus efficiently protected.
Two techniques for filling the volume separating the two components hybridized by the solder balls are known, the first one being known as “fast flow”, and the second being known as “no-flow”. Such techniques are for example described in document “Characterization of a No-Flow Underfill Encapsulant During the Solder Reflow Process”, of C. P. Wong et al., Proceedings of the Electronic Components and Technology Conference, 1998, pages 1253-1259.
The “fast-flow” technique follows the hybridization of the components. Particularly, during the hybridization, the components are submitted to the deoxidizing flux and heated up to a temperature higher than or equal to the melting temperature of the metal balls. Once the balls have been soldered, a cleaning of the deoxidizing flux present between the components is then implemented to avoid for the latter to create short-circuits between interconnects and to corrode them. Once the cleaning has been performed, the “fast-flow” technique then comprises depositing, on one or a plurality of edges of one of the components, liquid resin, that is, non-cured resin. The underfill resin then migrates by capillarity between the hybridized components and fills the volume separating them. The assembly is then submitted to a thermal treatment, or “curing”, to solidify the resin.
This technique is however very long due to the slowness both of the resin migration and of the thermal treatment. Further, the migration time increases according to the density of interconnects between the components, so that this technique is less and less adapted to the manufacturing of components having a high interconnect density.
The “no-flow” technique has been mainly developed to perform a fast filling of the volume between the hybridized components, and is now described in relation with the simplified cross-section views of FIGS. 1 to 3 in the context of a “flip chip” solder ball hybridization.
In a first step, coating resin 40 is deposited on a first electronic component 12a provided with solder balls 18a so as to cover them (FIG. 1).
In a second step, a second electronic component 12b, provided with solder balls 18b, is aligned on first component 12a, after which a pressure is exerted on the second component along the illustrated arrows by further raising the temperature of the assembly up to a temperature higher than or equal to the melting temperature of the metal forming balls 18a, 18b (FIG. 2). Balls 18a, 18b then bond to one another by thermocompression to form interconnects 42, resin 40 further occupying the volume between components 12a, 12b where interconnects 42 are housed (FIG. 3). Further, a heating being exerted to thermo-compress solder balls 18a, 18b, this heating is selected to activate the thermal treatment of resin 40 in order to cure it. As compared with the “fast flow” technique, it is thus not necessary to carry out a previous cleaning step, and a single heating step is implemented. There further is no migration of the resin, which is directly deposited at the location that it should subsequently occupy. The “no-flow” technique is thus fast.
Such a technique however has a number of disadvantages. First, as previously described, to form high-quality interconnects, the oxide layer covering balls 18a, 18b should be removed before their blending by thermocompression. For this purpose, the resin comprises a deoxidizing flux.
As known per se, the resin is a mixture of glue as a main component, for example, epoxy glue, and of a solvent which enables to adjust the viscosity of the resin and which is evaporated during the thermal treatment of the resin. The mixture may also comprise curing agents, particularly polymerizing agents, for example, a catalyst, a photoinitiator or a thermal initiator, and/or surface-active agents, for example, silane, which increases the bonding and the wettability of the resin on the surfaces of the components with which it has entered into contact, and/or particles for adjusting the thermal expansion coefficient of the resin, usually called “fillers”.
In the context of the “no flow”, deoxidizing flux is thus also incorporated in the resin to dissolve the oxide layer covering solder balls 18a, 18b. 
However, deoxidizing agents comprise ionic agents of high electric conductivity. Since they are present in the resin and cannot be removed therefrom, once the coating is over, such agents thus limit the electric resistivity of the resin, which may take values smaller than 1012 Ω/cm. Certain applications, particularly in cooled infrared detection, require resistivities greater than 2.1013 Ω/cm, thus making the “no-flow” technique unfit for such applications.